Second Edition
Book Title: Prescott's Principles of Microbiology
Authors: Joanne Willey, Kathleen Sandman
Topic: Anaerobic Respiration
Definition: Anaerobic respiration can occur when oxygen is not available, utilizing alternative terminal electron acceptors.
Common Acceptors Include:
Metals
Oxidized ions of nitrogen
Sulfur
Typical Environments:
Wetland soil
Human digestive tract
Uniqueness to Prokaryotes: Anaerobic respiration is exclusive to prokaryotic organisms.
Alternative Electron Acceptors:
Some bacteria utilize nitrate (NO3) reducing it to nitrite (NO2)
Others use sulfur compounds, reducing sulfate (SO42–) to sulfite (SO32–)
Electron Acceptor | Reduced Products | Examples of Microorganisms |
---|---|---|
Aerobic | O2, H₂O | All aerobic bacteria, fungi, and protists |
Anaerobic NO3 | NO2 | E. coli and other enteric bacteria |
NO2, N2O, N2 | Pseudomonas, Bacillus, Paracoccus spp. | |
H₂S | Desulfovibrio, Desulfotomaculum | |
CH4 | Methanogens | |
Acetate | Acetogens | |
Fe2+ | Various bacteria | |
Se, HSeO3 | Various bacteria |
Key Steps:
Nitrate (NO3) is reduced via Nitrate Reductase to nitrite (NO2)
Detection with sulfanilic acid (Nitrate Reagent A)
Further reduction leads to nitric oxide (NO) via nitrite reductase, then to nitrous oxide (N2O), and finally nitrogen gas (N2)
Testing Methodology:
Use of reagents to indicate presence of nitrite and final products (e.g., color change with Prontosil)
Process: Oxidation of hydrogen (H₂) to create a proton motive force.
H₂ is oxidized releasing protons (8H+) and electrons (8e)
Reduced Products: H₂S (hydrogen sulfide)
Electron Transport Chain Components:
Involves various enzyme complexes and electron carriers (e.g., cyt C3 and FADH) producing ATP from ADP
General Characteristics:
Utilizes electron carriers other than O2
Typically yields less ATP due to less positive reduction potentials compared to O2
Shorter Electron Transport Chain:
Less hydrogen pumped out
Dissimilatory Nitrate Reduction:
Nitrate is used as a terminal electron acceptor; not available for cellular assimilation
Denitrification:
Converts nitrate to nitrogen gas causing loss of soil fertility
Example Organism: Paracoccus denitrificans
Differences Compared to Aerobic ETC:
More complex and branched
Different electron carriers used
Denitrification Steps:
Nitrate → Nitrite
Nitrite → Nitric Oxide
Nitric Oxide → Nitrous Oxide
Nitrous Oxide → N2
Dissimilatory Nitrate Reduction:
Conversion of nitrogen allowing cellular use for amino acids and nucleotides.
Denitrification:
Release of nitrogen gas (N2) into the atmosphere, contributing to nitrogen cycles and soil health.